Organic light emitting elements have come to attention as thin luminous materials.
Organic light emitting elements utilizing electroluminescence (EL) of organic materials (the so-called organic EL elements) are completely solid elements in the form of a thin film which can emit light at low voltage of about several volts to several tens of volts. These organic light emitting elements have many excellent characteristics, such as high luminance, high luminescent efficiency, thinness, and lightness in weight. These characteristics of the organic light emitting elements receive attention in applications to backlights for a variety of displays and planar luminous elements, e.g., light sources for illumination used in display boards, such as signs and emergency lights.
A typical organic light emitting element includes two electrodes and a luminous layer composed of an organic material and disposed between the electrodes, through which luminescent light generated in the luminous layer is extracted to the outside. At least one of the two electrodes should be transparent to extract the luminescent light.
The organic light emitting elements can generate light with high luminance at low electricity, and are advantageous in high visibility, high response rate, prolonged service life, and low power consumption. Unfortunately, the use efficiency of the light generated in the organic light emitting element is at most 20%, which indicates significant loss of the luminescent light inside the element.
FIG. 19 is a schematic sectional view showing a conventional organic light emitting element.
An organic light emitting element 100 includes a metal electrode 101, an organic luminous layer 102 having a refractive index of about 1.8, a transparent electrode 103 having a refractive index of about 1.8, and a transparent substrate 104 having a refractive index of about 1.5, which are sequentially laminated from a lower layer in the drawing. In the drawing, the arrows 110a to 110e indicate characteristic light components of the light generated in the organic luminous layer 102.
The light component 110a is perpendicular to the organic luminous layer 102 as a light emitting surface, and is extracted through the transparent substrate 104 from a light-emitting side (to the air).
The light component 110b is incident on the interface between the transparent substrate 104 and the air at the critical angle or less, and inflects at the interface between the transparent substrate 104 and the air to be extracted from the light-emitting side.
The light component 110c is incident on the interface between the transparent substrate 104 and the air at an angle larger than the critical angle. The light component 110c is totally reflected at the interface between the transparent substrate 104 and the air, and cannot be extracted from the light-emitting side. This loss due to total reflection of the light is referred to as “substrate loss,” which is typically about 20%.
The light component 110d is incident on the interface between the transparent electrode 103 and the transparent substrate 104 at an angle larger than the critical angle and satisfies the resonant condition. Such a light component 110d is totally reflected at the interface between the transparent electrode 103 and the transparent substrate 104 to generate a waveguide mode, in which the light component is confined within the organic luminous layer 102 and the transparent electrode 103. This loss due to the waveguide mode is referred to as “waveguide loss,” which is typically about 20 to 25%.
The light component 110e is incident on the metal electrode 101, and reacts with free electrons in the metal electrode 101 to generate a plasmon mode, one of the waveguide modes, in which the light component is confined near the surface of the metal electrode 101. This loss due to the plasmon mode is referred to as “plasmon loss,” which is typically about 30 to 40%.
As described above, the conventional organic light emitting element 100 has substrate loss, waveguide loss, and plasmon loss; hence, light emitting elements are faced with the task of emission of a larger amount of light by reduced extraction loss.
For example, PTL 1 discloses an organic electroluminescent (EL) device including a light scattering unit composed of a lens sheet and disposed adjacent to a light extraction surface.
PTL 2 discloses a substrate for a light emitting device disposed on the light emitting surface of a light emitting device, the substrate including an irregular layer having a high refractive index of 1.6 or more and an average surface roughness of 10 nm or more on at least one of surfaces of the substrate and one or more substrate layers having a refractive index of 1.55 or more, and a light emitting device.
Irrespective of the problems described above, the organic light emitting elements have an advantage over the conventional light emitting elements, that is, planar light emission from a thin film To utilize this advantageous feature, the organic light emitting elements should be formed on flexible transparent substrates (supports). To meet such a requirement, an enhancement in heat resistance of flexible transparent substrates has been examined, and there is still a great demand for techniques of forming high-quality organic light emitting elements on flexible transparent substrates, such as PET films, that have been extensively used in the market.
Unfortunately, these transparent substrates have low heat resistance, and desired techniques are yet to be developed due to technical difficulties. Although internal light extraction (waveguide-mode light) structures have come into focus in enhancing the luminescence efficiency and the durability of the organic light emitting elements, materials and processes should be still developed to achieve high performance of organic light emitting elements formed on flexible transparent substrates having low heat resistance, such as PET films.